Greenhouse Gases and Climate Change (C9.2)
The greenhouse effect (natural)
The natural greenhouse effect keeps Earth at a life-supporting average of ~15°C (without it, Earth would be ~−18°C).
Mechanism:
- Short-wave radiation (UV/visible light) from the Sun passes through the atmosphere and warms the Earth's surface.
- Earth's surface emits longer-wave infrared (IR) radiation.
- Greenhouse gases (GHGs) in the atmosphere absorb some of this IR radiation.
- GHGs re-emit IR in all directions, including back to Earth → warming effect.
Greenhouse gases:
- Water vapour (H₂O) — largest natural contributor
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Nitrous oxide (N₂O)
- Ozone (O₃)
Enhanced greenhouse effect (human-caused)
Human activities have increased the concentrations of GHGs:
| Gas | Main human source |
|---|---|
| CO₂ | Burning fossil fuels, deforestation, cement production |
| CH₄ | Livestock (enteric fermentation), rice paddies, landfill, natural gas leaks |
| N₂O | Fertilisers, industrial processes |
Increasing GHG concentrations → more IR absorbed → Earth warms more than naturally → global warming.
Evidence for global warming
- Temperature records (global average rising)
- Ice core data (CO₂ levels correlated with past temperature)
- Sea level measurements (rising)
- Glacier retreat
- Changes in species distribution
Consequences of climate change
- Rising sea levels (ice melt + thermal expansion of oceans) → coastal flooding
- More extreme weather events (storms, droughts, floods)
- Habitat loss and species extinction
- Food security threats
- Ocean acidification (CO₂ dissolves in seawater → carbonic acid → harms corals and shellfish)
Carbon footprint
A carbon footprint is the total amount of greenhouse gases (expressed as CO₂ equivalent) produced directly or indirectly by an individual, organisation, event or product.
Reducing carbon footprint:
- Use renewable energy (solar, wind, tidal)
- Improve energy efficiency (insulation, LEDs)
- Eat less meat/dairy
- Use public transport/electric vehicles
- Carbon capture and storage (CCS)
- Afforestation (planting trees)
Common exam errors
- Saying greenhouse gases "trap heat" — they absorb and re-emit IR radiation; heat is not literally trapped.
- Confusing the ozone hole (UV protection) with the greenhouse effect (IR absorption) — they are different problems.
- Saying CO₂ reflects sunlight — it absorbs IR (emitted from Earth's surface), not incoming solar radiation.
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